Attenuating apparatus for highfrequency energy



July 18, 1950 J. v. HUPCEY 2,515,228

' ATTENUATING APPARATUS FOR HIGH-FREQUENCY ENERGY Filed Ma y 2a, 1946 INVENTOR J08 HUPCEYI ATTORNEY Patented July 18, 1950 ATTENUATING' APPARATUS FUR HIGH- FREQUENCY ENERGY;

Joseph V. 'HupceigiWest Hempstead, N; Y., as-

signor to The: Sperry Corporation, a corporation.

-- of'Delawa-re Application May. .28, 1946, Serial N0; 672,751

8C1aiins. (or. us -44') The present. invention. relates. to. improvements in attenuating apparatusv for ultra-highfrequency energy.

Ultra-high-frequency energy (thatis energy of wavelengths of. one meter .orshorter) is generally conductedby meansoi ahollow conductor of either the coaxial line or wave guide type of, construction. For measuring. and; testing, purposes, there is a need for simple. apparatus'that can be interposed along the hollow conductorto attenuate the energy in the conductor- (that istored'uce its energy level) by adjustable and known amounts. One known kind of apparatus. makes useof a wave guideanda rounded cardof resistive. material inserted to an adjustable degree into the wave. guide, in the portion of the.el'ectromagnetic field within the guide having the greatest voltage gradient. I

It would be desirable toihave similar simple attenuating apparatus for coaxial lines. In a coaxialline. operating in the conventional transverse electromagnetic (TEM) mode, the electric field is radial' with respect to the innericonductor. However, radial insertion of a resistance card with a rounded edge permits only a small area of i the resistance. cardtoibe put in the intensity portion of the field. (which is'close tcthe inner conductor) even at maximum insertion; because the amount of insertion 'is'limitedby'tl'iexpresence of the inner conducton. Therefore, even at maximum insertion; there is a" very limited amount of attenuation.

Accordingly, it is an'obje'ct'of the" present invention to provide simple attenuating apparatus for high frequency energy in' coaxiallines that overcomes the above defects and eif'ects comparatively high" amounts of attenuation.

Another object is to providesimpleattenuating apparatus for high frequency energyin coaxial lines that minimizes energy reflections, while eifecting comparatively higham'ounts of attenuation.

Yet another object is toprovide simplevar'iable attenuatingapparatus for" high'fi'equen'cy energy in coaxial li'nes that' minimizes-variation in reflected energy with variation in attenuation,

while permitting acomparatively largemaximum value of attenuation.

A further object-isto provide simple attenuating apparatus for high frequency ener'gy in transmission lines having" inner and outer conductors that provide minimunivariation of reflected energy with variation in"frequency' of the applied energy, while permittingwoniiaaratively large amounts of attenuation;.andk'whioh is useful over a wide r range of frequencies.

.Aiurther object .is. toprovide variable coaxial line. attenuating apparatusthat provides a. substant'iall y, linear relation between. insertion of attenuating material into the. coaxial, linezand amountof attenuation effected. bythe attenuating, material. i 1

Theinventionin another, of its aspects relates to. novel features of, the instrumentalities described hereinfor achieving theprincipal objects of the inventionand to. novel principles employed inthose instrumentalities, whether or. not. these features andv principles are. used .for the Jsaid principal objects orin the said. field.

A further object of the. invention. is to. provide improved apparatus. and .instrumentalities embodying. novel featuresand principlesadapte'd for use in realizingthe above objects and also adapted for use in other field's. 1

These. and other objects are achieved by the provision of attenuating apparatus that comprises va sectionof a coaxial transmissionlineiandmeans through" the high intensity part' of the field, thereby securing a wide range of attenuation.

At thesame time the construction of" the present invention minimizes energy refie'ction";;mini mize's' variations in the" amount of" energy' reflection with variation of amount of attenuation; minimizes variation" of amount: of energy reflection with variation of the frequency of" the energy; and provides an approximatelinear relation between amount of. insertion of the resistanceinto-the transmission line anddecibels of attenuation produced.

Other objects and advantages will become apparent from the specification, taken in connection with the accompanying drawings wherein:

Fig. 1 is an elevation view, with some parts in cross-section, of one form of the invention;

Fig. 2 is an elevatioi'r'view;v partly incomes section, taken along: liner2'2.: of Figs L. 1

Fig. 3 is a view'in crossesectionyto anienlarged scale, of the coaxial line of 'l',.-.showing -.the electric field lines in the" conventional) transverse electromagnetic (TE'MD mode ois Q emtmmanu the relation of the resistance card to both the field lines and the inner conductor;

Fig. 4 is a fragmentary elevation view in crosssection of another embodiment showing a modification of the apparatus below line 12-11 of Fig. 2;

Fig. 5, is a fragmentary plan view ofthe inner conductor loft he apparatus of Fig. 4, showing the rectangular configuration of the slot in the inner conductor; and

Fig. 6 is a similar view showing a modification of the configuration of the slot of Fig. 5.

Referring to Figs. 1, 2 and 3, the apparatus em-. bodying the present invention includesasection of coaxial line 4 comprising an'outer conductor 5 and an inner conductor 6. A resistance card 1 4 In the section of line 4 in which the shunt conductance per unit length of line 4 is gradually increased, the characteristic impedance of infinitesimal lengths of the section is gradually decreased as the shunt conductance is gradually increased. Accordingly, it maybe said that the section oi "resistance card] that produces a gradually increasing shunt" conductance per unit length of line 4 along the axis of line 4, also pro- 10' *p'edance along the'axis.

' -mrhe transverse electromagnetic (TEM) mode of propagation of energy along line 4, the intenduces a gradually decreasing characteristic imsityi ofjthe electric component of the electromagformed of attenuating material upon a non-conductive carrier, such as provided by a resistive coating 8 upon a fiat laminated Bakelitebase pr carrier 9, is carried on a guide member ID connected toa rack II, and passes through, an axial slot'l lin outer. conductor 5. The rackll enjgagesa pinion 12 that is operable by a knob I3 to adjust, the amount of insertion of the resistance card I through slot M. The path of the insertion is along a non-diametric chord that is displaced a small distance, such as of the order 'of .0 0 5 inch, from the inner conductor 5, as shown inFigs. 2 and 3. It is thus substantially tangential to the inner conductor. A shielding enclo- "sure l5 is'placed about the rack I I, pinion l2, an'd ,slot I 4 ,to prevent electromagnetic energy from radiating into space from coaxial line 4. Also included in the apparatus are conventional onequarter-wavelength stub supports l6 and H for supporting inner conductor 6 coaxially of outer conductor 5, and suitable coupling means IB and I9 for connecting the apparatus to other circuit elements. I

When resistance card I is inserted into the coaxial line 4, the shunt conductance per unit lengthof the portion of line 4 containing the card '1 is increased by an amount dependent upon the degree of insertion. Because of this increase in shunt conductance, the attenuation eifected by that section of the coaxial line 4 is increased, and

desirable attenuating action is produced.

The edge 20 of the resistance card is rounded inorder to minimize reflections of energy by the resistance card. Because of the rounded edge 20, the shunt conductance per unit length of the transmission line 4 is gradually increased along its axis from the shunt conductance of the line 'in. advance of the resistance card to the point 'where the increase of shunt conductance effected by the resistance card 'I is maximum and then is gradually decreased from that maximum point to the shunt conductance of the line 4 beyond the resistance card-1. Thus, there is no abrupt Th'e characteristic impedance of a coaxial .line is a function of the shunt conductance perunit length of line, as can be seen from the formula Zo=characteristic impedance of the line R=series resistance per unit length L=series inductance per unit length G=shuntconductance per unit length C=shunt capacitance per unit length ,i

5W1=21r times the frequency -(F).-of-.'opieration.i-

change in unit shunt conductance alongthe'axis ,of the line 4 and, therefore, energy reflection by .the resistance .card 1 is minimized.

tance from the inner conductor. seen inFi'gil, which shows the apparatus in a 'netic fieldfdecreases with increase of radial dis- As it can be positionfor a large amount of attenuation, a comparatively large area of the card I is close to the inner conductor Bin that position. Being close tothe inner'conductor 6, this comparatively large area of resistancecard is in the high intensity field, and, therefore, produces a large amount of attenuation.

The path of the insertion of the resistance card I may be substantially tangential to the inner conductor. In this format maximum insertion,

a large area of the resistance card would also be inth'e high intensityfield close to the inner conductor. This would result, however, in physical contactlbetween the resistance card I and inner conductor fiand in an undesirable rubbing action between the two during adjustment of the card lby knob 13. vIt is advisable, therefore, to have the resistance card] as close as possible to inner conductor 6 without actual'contact in order that it may be in high intensity field.

, In actual use, a variable attenuator of ,thetype shown in Figs. 1-3 was connected between .a

matched loadand a source of super-high frequency energy, variable in wavelength from 5.0 to 7.2 .cm. A standing wave detector was con- .nected between thesource and the variable attenuator, and power-measuring apparatus was source and the attenuator apparatus were measured for various degrees of insertion of the card and for various, wavelengths over the wavelength bandof 5.0 to 7.2- cms.

It was found that the average of the voltage standing wave ratios was less than 1.112 for wavelengths from 5.0 to 6.3 cms. and less than 1-.25 for wavelengths 6.4 cm. to 7.2 cm. The maximum attenuation effected over the same band of frequencies; varied only 2.5 decibels. It was also found that there was 'an approximately linear relationship between attenuation in decibels and degree of insertion of the resistance card 1 into theline 4.

It will be understood that the values just given represent'only one set of operating conditions, and-many-other values and dimensions of the parts of the structure may be used. Thus, other values of,, resistance cards may be used. .Generally,,it ,may.be stated thatjhigher values of re,-

sistance produce smaller maximum amounts of attenuation, while causing lower standing wave ratios; and, conversely, lower values of resistance produce larger values of attenuation while causing higher standing wave ratios.

In the modification shown in Fig. 4, the attenuating card 26, similar to card I, is inserted radially into the electric field. It will be understood that the structure shown in Fig. 4 is intended to replace that portion of Fig. 2 which is below line aa thereof. The advantage of wide range of attenuation found in the embodiment of Figs. 1-3 is present here also.

In Fig. 4, coaxial line 2| comprises an outer conductor 22, having an axial slot 23 therein, and an inner conductor 24 having a rectangular axial slot 25 preferably diametrically therein, as shown in Fig. 5. The attenuating material, in the form of resistance card 26 having a rounded profile as in Figs. 1-3, is inserted through the slot 23 in the outer conductor 22 along a path passing through the slot 25 in the inner conductor 24. For small values of attenuation, card 26 is inserted only slightly into slot 23, and its lower edge remains out of slot 25. For higher values of attenuation, card 26 is inserted further through slot 23, even into slot 25. For maximum attenuation, card 26 extends substantially fully across a diameter of outer conductor 22.

As in Figs. 1-3, a comparatively large area of the resistance card 26 may thus be positioned in the high intensity portions of the field close to the inner conductor, at maximum insertion. Therefore, a comparatively large amount of attenuation is obtainable in this form of the invention, also. This variable attenuator was connected to the auxilary apparatus described above in the tests of the embodiment of Figs. 1-3. The results showed that voltage standing wave ratios fluctuated between higher upper and lower limits than with the embodiments of Figs. 1-3, for the same frequency and attenuation ranges.

Referring to Fig. 6, there is shown a modification of the apparatus of Figs. 4 and 5. In this form, a slot 21 of substantially rectangular shape is formed in inner conductor 28, but its ends 29 and 30 are tapered axially. Because of the taper, there is a more gradual change in characteristic impedance between the coaxial line section before the slot and the line section containing the slot; and also between the latter section and the line section beyond the slot. This serves further to reduce standing wave ratios in the system.

I have therefore provided simple attenuating apparatus for high frequency energy in coaxial transmission lines that produces comparatively large maximum amounts of attenuation with only minimum amounts of energy reflection and with maximum band-width.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. High frequency apparatus comprising a transmission line having an inner conductor with an opening therein and an outer conductor with an opening therein, attenuating means, and means for adjustably inserting said attenuating means into said line through said opening in the outer conductor and along a path passing through said opening in said inner conductor.

2. Apparatus as in claim 1 wherein the opening in said inner conductor has a tapered portion at each end of the opening.

3. Apparatus for adjustably attenuating high frequency energy comprising a high frequency transmission line including an outer conductor and an inner conductor, said outer conductor having a longitudinal opening therein, a resistance card with a rounded edge and comprising a non-conductive base and a resistive coating on said base, and means for slidably adjusting said resistance card into said transmission line through said opening and in a plane substantially tangential to said inner conductor.

4. Apparatus for adjustably attenuating high frequency energy comprising a high frequency transmission line including an outer conductor with an opening therein and an inner conductor with an opening therein, a resistance card with a rounded edge and comprising a non-conductive base and a resistive coating on said base, and means for adjustably sliding said resistance card into the transmission line through the opening in said outer conductor and in a plane passing through both said opening in the outer conductor and said opening in the inner conductor.

5. High frequency apparatus comprising a transmission line having an inner and an outer conductor, a card-shaped resistance element, and means for inserting a portion of said resistance element into a longitudinal slot in said transmission line a distance transverse to the longitudinal axis of the transmission line which is greater than the distance between said inner and outer conductors, the longitudinal axis of said resistance element being coextensive with the longitudinal axis of said transmission line.

6. The apparatus of claim 5 wherein the portion of said card-shaped resistance element which is inserted into said transmission line has a rounded edge.

7. The apparatus of claim 5 wherein said resistance element is inserted into said transmission line along a plane which is substantially tangential to said inner conductor.

8. The apparatus of claim 5 wherein said longitudinal slot comprises aligned slots in said inner and outer conductors and said resistance element is inserted into said transmission line along a. diametrical plane which passes through said slots.

JOSEPH V. HUPCEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS.

Number Name Date 1,957,538 Jensen May 8, 1934 2,399,930 Keister May 7, 1946 2,401,296 Fernsler June 4, 1946 2,409,599 Tiley Oct. 15, 1946 2,414,785 Harrison et a1. Jan. 21, 1947 2,426,992 Folland Sept. 9, 1947 2,452,737 Eisenstein et a1 Nov. 2, 1948 OTHER REFERENCES A. I. E. E. Technical paper 46-40, published January 1946 by American Institute of Electrical Engineers, 33 West 39th St., New York City. Page 22 and the page containing Figure 23 (page 33) relied upon. 

